Theory

After olsrd is running for a while, a node knows about the existence of every other node in the mesh cloud and which nodes may be used to route traffic to them. Each node maintains a routing table covering the whole mesh cloud. This approach to mesh routing is called proactive routing. In contrast, reactive routing algorithms seek routes only when it is necessary to send data to a specific node.

There are pros and cons to proactive routing, and there are many other ideas about how to do mesh routing that may be worth mentioning. The biggest advantage of proactive routing is that you know who is out there and you don't have to wait until a route is found. Higher protocol traffic overhead and more CPU load are among the disadvantages. In Berlin, the Freifunk community is operating a mesh cloud where olsrd has to manage more than 100 interfaces. The average CPU load caused by olsrd on a Linksys WRT54G running at 200 MHz is about 30% in the Berlin mesh. There is clearly a limit to what extent a proactive protocol can scale -depending on how many interfaces are involved and how often the routing tables are updated. Maintaining routes in a mesh cloud with static nodes takes less effort than a mesh with nodes that are constantly in motion, since the routing table has to be updated less often.